eyeballing; measuring by eye; rough estimate by sight
Refers to estimating a quantity, size, or amount by sight without using measuring tools. Often used in cooking, DIY, or when precision is not required.
料理は目分量で味付けをすることが多い。
I often season dishes by eye.
I added the ingredients by rough estimate, and it turned out well.
Compound of 目 (eye) and 分量 (amount, quantity), literally 'eye amount'.